The Ultimate Member WordPress plugin suffers from a stored XSS vulnerability due to improper neutralization of user-supplied input in the 'um_loggedin' shortcode. Authenticated users with contributor or higher privileges can inject arbitrary JavaScript that executes in the context of other users viewing the injected page. This vulnerability affects all versions up to and including 2.8.6. The CVSS 3.1 base score is 6.4, reflecting network attack vector, low attack complexity, low privileges required, no user interaction, and impacts on confidentiality and integrity with no availability impact.

An attacker with contributor-level access or above can inject persistent malicious scripts into pages using the vulnerable shortcode. These scripts execute in the browsers of users who visit the compromised pages, potentially leading to session hijacking, data theft, or other unauthorized actions within the context of the affected site. The impact is limited to confidentiality and integrity, with no direct availability impact reported.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is released, restrict contributor-level access to trusted users only and consider disabling or avoiding use of the 'um_loggedin' shortcode. Monitor for plugin updates from Ultimate Member and apply official patches promptly once available.